Proceedings of COBEM 2003
COBEM2003 - 0294 Copyright © 2003 by ABCM
17th International Congress of Mechanical Engineering
November 10-14, 2003, São Paulo, SP
INFLUENCE OF AMBIENT TEMPERATURE IN COMBINED CYCLE
POWER PLANT PERFORMANCE
Felipe Raúl Ponce Arrieta
Thermal Systems Study Group (NEST), Federal University of Itajubá (UNIFEI), Av. BPS 1303 Caixa Postal 50, Itajubá, MG,
Brasil, CEP 37500-903, Tel-Fax: +55 35 3629 1355
e-mail:[email protected]
Electo Eduardo Silva Lora
Thermal Systems Study Group (NEST), Federal University of Itajubá (UNIFEI), Av. BPS 1303 Caixa Postal 50, Itajubá, MG,
Brasil, CEP 37500-903, Tel: +55 35 3629 1321, Fax: +55 35 3629 1355
e-mail: [email protected]
Abstract. Thermal power plants have become an important issue as part of energy supply systems, mainly because of the need of
diversified power generation systems and the availability of natural gas, the main fuel used in this type of energy generating system.
With the implementation of the Priority Plan of Thermal Power Plants in the Brazilian scenario, dozens of units will be installed
which will make it possible to transform the Brazilian Electric System, today mainly based in hydraulic principles, into a hydrothermal system. The operation of a combined cycle thermal power plant is influenced by the conditions that are present at the place
where it is installed, mainly ambient temperature, atmospheric pressure and the air relative humidity. Those parameters affect the
generated electric power and the Heat Rate during the operation. Among these variables, the ambient temperature causes the
greatest performance variation during the operation. That is the reason why the influence of this variable on this type of generating
unit is studied. The plant selected for this study has a multiple shafts configuration and is composed of two Siemens-Westinghouse
501F gas turbines, coupled to three level pressure HRSGs and re-heating with supplementary firing and a steam turbine. The most
relevant results obtained from a thermodynamic simulation, in which the Gate Cycle Software version 5.40.0.r was used, are the
curves of generated power, as well as, Heat Rate and thermal efficiency as a function of ambient temperature and the supplementary
firing after burning temperature.
Keywords. Electric power generation, operation, thermodynamic analysis, efficiency, Heat Rate.
1. Introduction
Thermal power plants began to gain strength in the country after the need of diversifying the generating park and
the availability of natural gas, which is the main kind of fuel used by this type of generating unit. The implementation of
the Priority Thermoelectric Program will result in the installation of several thermal power plants. This will make the
Brazilian Electric System become hydrothermal rather than predominantly hydrological.
It is important to highlight the fact that the behavior and he operation of thermal power plants is considerably more
complex than the operation of hydroelectric plants because of the use of working fluids at high temperature and
pressure and the consequent difficult operational conditions of the tubes metal, heating surfaces, the turbine combustion
chamber, casing, headings, etc. In addition, one must also consider the influence of metals corrosion and erosion caused
by different elements, the need of complex automatic control systems, the need to implement system and equipment for
pollution control, and the constant effort for maintaining the operation high efficiency and reliability, etc.
Currently, the use of natural gas for thermoelectric generation considering its cost is justified, in most of the cases,
at high efficiency installations, which are typical of combined cycle thermal power plants. In this type of generating
unit, the operation problems pointed out in the above paragraph manifest themselves in a relevant way, and they are
aggravated as a consequence of the presence and interconnection of the main components: gas turbine, HRSG and
steam turbine. It is important to highlight that besides these operational problems, those units, designed at ISO
conditions (15 °C of ambient temperature, 101,32 kPa of atmospheric pressure and 0,6 of air relative humidity) are
extremely sensitive to changes in ambient conditions. The main reason for this sensitivity is the influence of these
parameters on off-design gas turbine operation (ambient temperature, atmospheric pressure, relative humidity are
different from ISO conditions), where an average of 2/3 of the plant’s total power is generated. In this sense, as a general
rule, it is possible to say that combined cycle thermal plants with similar configurations present similar off-design
behavioral trends, although different capacities have been projected. (Kehlhofer, et al., 1999).
While the air temperature increases in relation to the project temperature, the power generated by the plant
decreases. This is explained by the considerable influence that this parameter has on the power generated by the gas
turbine.
According to Kehlhofer, et al. (1999) the gas turbine is designed to operate with a constant air volume in the
compressor. When the ambient air temperature increases, its specific mass is reduced. In order to ensure the same air
volumetric flow, the mass flow is reduced, causing the power of the gas turbine and the amount of heat generated in the
HRSG to fall. The influence of the atmospheric pressure is related to the air density variation. For low pressures, that is,
high altitudes in relation to the sea level, the air density is decreases. Disregarding the pressure losses in the gas turbine
inlet and outlet ducts, and considering that the efficiency of the steam cycle does not change (facts that happen in a real
context), the combined cycle plant presents a behavior that is similar to what was previously explained for ambient
temperature. It is important to highlight that the effect of the ambient pressure on the performance must be considered,
mainly during the project phase, for once the plant is installed, the variations of this variable are neglected. While the air
relative humidity increases, the power generated by the combined cycle plants also increases, considering the other
parameters to be constant. In this case, the gas turbine efficiency is slightly reduced, as well as its power. However, the
temperature of the gas turbine exhaust gases rises, and therefore the power generated by the steam cycle is increased.
The final result depends on which of the factors described above is predominant, but in both cases, the total power
variation is very small. Plants presenting cooling towers deserve special attention. In these plants, the air relative
humidity is directly related to the level of vacuum in the condenser, and consequently to the temperature of the steam
turbine exhaust steam. In these cases, a lower air relative humidity results in a greater vacuum and higher efficiency.
The analysis that was presented allows the supposition that out of these three variables, the ambient temperature has
greater influence on the off- design operation of a combined cycle plant. This way, the main goal of this study is to
evaluate the influence of this variable on the operation of this type of installation. In order to carry out this study, the
values of the other variables were keep constant, for example, atmospheric pressure, air relative humidity, electric
energy frequency, power factor, fuel characteristics and its quality, etc.
2. Assumptions
This item describes the suppositions adopted to evaluate the influence of the ambient temperature on the operation.
The simplified thermal scheme of the installation selected for the analysis is shown in Figure 1.
Figure 1 – Combined cycle plant simplified thermal scheme
The thermal scheme main characteristics are:
• Two Siemens-Westinghouse 501F gas turbines;
• Two HRSGs presenting three pressure levels and fuel supplementary firing. A detailed scheme of the HRSG is
displayed in Figure 2. According to this figure fuel supplementary firing takes place after the two final highpressure superheating stages. The first re-heating stage and the first high-pressure superheating stage are placed
after the supplementary firing;
• A high, intermediate and low-pressure steam turbine, and the last one presents divided end flow
• Dearator condenser with a cooling system that has wet towers cooling system;
• Cooling system pumps: The low-pressure pump at the condenser outlet and the high-pressure pump is
responsible for elevating the water pressure to high and intermediate levels;
• Natural gas supply: The fuel that will be used by the gas turbines is heated, but the fraction that will be used for
the supplementary firing is not.
Figure 2. HRSG detailed scheme
Figure 3 illustrates the thermal scheme of a combined cycle drawn by using Gate Cycle version 5.40.0.r. This figure
also presents the results of the design point simulation.
02 GT - 501F Siemens-Westinghouse + 01 Steam Turbine
S26
S44
S43
M2
S62
S12
S13
S52
S30
S10
V3
S42
SPHT7
SPHT6
S45
S46
S32
103.81 P T 76.44
S59
V4
S34
S27
S11
S58
S61
S54
SP5
ECON3
S1
S2
DUCT1
S3
SPHT1
S28
SPHT4
S4
SPHT2
S5
DB1
S29
SPHT5
SPHT3
S6
S50
S7
M3
SP3
EVAP1
S56
SP4
ECON2
S49
S23
GT1
S51
M4
S53
SPHT9
173.48
S31
S39
SP1
EVAP2
ECON4
S33
S57
M1
S8
EVAP3
S35
S60
S55
SPHT8
GT1 NG
S69
S47
MW
446.52W H 64.75
ECON1
S24
S36
S41
S21
V1
S66
S63
S14
S37
S40
S38
S25
V2
S48
M6
SP2
PUMP3
S70
S68
V5
HX1
S67
S64
S136
V12
S65
MODEL:
2TG1TV
S131
S99
CASE:
S142
ST4
M13
ST5
M14
S15
ST6
M12
S126
S130
CT2
S121
2TG1TV
MU1
S19
POWER:
Natural gas
V14
S133
CND2
S134
S16
V11
SP11
S143
PUMP6
600.51
S22
S135
HDR1
S20
HR:
6829.55
EFF:
52.71
S116
S125
S17
S132
SP12
S9
PUMP4
S111
S122
S18
V13
S120
M11
S129
S127
V6
M10
S102
S123
S119
SPHT15
S114
S86
S112
S113
S71
DUCT2
S72
S73
SPHT10
SPHT11
S74
SPHT12
S75
DB2
S76
S77
SPHT13 SPHT14
S78
EVAP4
S84
S79
M7
SP6
S85
ECON5
S90
SP7
M8
S87
S83
S81
SP9
S89
EVAP5
S91
103.81 P T 76.43
446.52W H 64.75
S96
S110
M9
SP8
S93
S101
S94
ECON7
S97
S98
EVAP6
S95
ECON9
S105
S138
SPHT18
SPHT16
S139
S100
S103
S92
S88
ECON6
GT3
V7
S104
SPHT17
S82
S80
S124
S115
173.47
MW
S118
S128
S117
V8
GT1 NG
S107
S108
V9
S106
SP10
PUMP5
S109
S137
V10
HX2
S140
Suplementary firing NG
S141
Figure 3. Thermal scheme of a combined cycle plant and the results of the design point simulation
The most important parameters of the installation described above are presented in Table 1 and they refer to
parameters used for the thermodynamic simulation at design point.
It is possible to observe that the data shown in Table 1 are divided in four main groups. In the first group, besides
the ISO parameters, it is established that the net total electric power generated by the plant at design point is 600 MW.
The second group of data defines the fuel used during the simulation, as well as its Low Calorific Value and the
supplying conditions. The third group is dedicated to the gas turbine. In this case, the data presented had already been
already implemented in the Gate Cycle software when the turbine was selected. The last group is related to the steam
cycle and the plant’s auxiliary systems. The following values can be observed: steam pressure and temperature for
different levels of pressure presented by the HRSG, condenser operating pressure, efficiency of the steam turbine three
stages and the imposition of a minimum steam at the low pressure turbine outlet, which is based on technical criteria.
The other data that were shown are related to the fuel supplementary firing, the condenser cooling system, the pumps
and the BOP power consumption. It is important to highlight that the temperature of the gases after the fuel
supplementary firing was limited at 675 °C aiming at avoid the formation of steam in the final section of the
economizer tubes.
The final suppositions are related to the calculation of priorities during the simulation. In this sense, water
proprieties and steam were determined according to Reynolds (1979), whereas an ideal gas behavior was considered for
the gases, and the proprieties were calculated according to Chase Jr. (1998).
Table 1. Main data for the simulation of the combined cycle thermal plant at design point
Parameter, unit
Ambient Temperature, °C
Atmospheric pressure, kPa
Relative humidity
Net total electric power, MW
Fuel:
Natural gas, PCI, kJ/kg
Supply conditions, MPa/°C
Siemens-Westinghouse 501F gas turbines (a):
Gross power, MW
Compression isentropic maximum efficiency
Combustion efficiency
Turbine isentropic efficiency
Electricity generator efficiency
Turbine inlet temperature, °C
Turbine outlet temperature, °C
Cooling air fraction
Auxiliary power consumption, MW
Steam cycle with three levels of pressure and re-heating
High pressure steam, MPa/°C
Intermediate pressure steam and re-heating, MPa/°C
Low pressure steam, MPa /°C
Condenser operating pressure, kPa
HRSG (b):
Additional burning temperature, °C
Additional burning efficiency
Minimal gas exiting temperature, °C
Heat transfer coefficient, kJ/s-m2-K
Three-stage steam turbine:
Net power with additional burning, MW
High pressure turbine isentropic efficiency
Intermediate pressure turbine isentropic efficiency
Low pressure turbine isentropic efficiency
Minimum quality at the out let of the low pressure steam turbine (c)
Total electromechanical efficiency
Condenser:
Heat exchange area, m2
Heat transfer global coefficient, kJ/s-m2-K
Cooling tower:
Capacity, kJ/s
Number of fans
Fan total power, kW
BOPs:
Pump isentropic efficiency
Total losses referring to the steam turbine power
Value
15
101.32
0.60
600
46515
2.758/15
174.66
0.90
0.99
0.9431
0.985
1382.5
608
0.178
1.18316
15.6/530
3.2/530
0.75/305
5.06
675
0.976
70
0.45426
253.57
0.8098
0.9259
0.8867
0.85
0.94464
30528
2.85
421418
10
960.5
0.75
0.0198
Notes:
a) Source: Gas Turbine World Handbook (1998);
b) The temperature of the gas after the additional burning was limited at 675 °C aiming at avoiding the formation
of steam in the final section of the economizer tubes during the off-design operation. The pressure drop of the
gas and of the working fluid (water-steam) was not considered;
c) According to Boyce (1999) this value avoids blades erosion within the last stages of the turbine.
3. Methodology
This item intends to explain the criteria used to evaluate the influence of the ambient temperature on the operation
and performance of a combined cycle plant and the different stages in which the study was carried out. The particular
case of the installation presenting the characteristics shown previously, it is not possible to disregard the effect of the
fuel supplementary firing on the plant’s performance. By using the fuel supplementary burning in the HRSG, it is
possible to mitigate the power loss in the gas turbines caused by a rise in the ambient temperature, affecting the
installation’s thermal efficiency at the same time. The power loss in the gas turbines is compensated by the
supplementary firing, for it is possible to generate more steam in the HRSG, consequently, more power in the steam
turbine. This way, aiming at accomplishing this study’s goal, a parametric study involving two variables: ambient
temperature and gas temperature after supplementary firing, was carried out.
Table 2 shows the values of the variables that were used for the parametric study, that is, for each value of ambient
temperature that was considered, the plant’s performance was evaluated by using different gas temperature values after
the supplementary firing in the HRSG.
Table 2. Parametric study variables and their values.
Variable
Ambient temperature, °C
Gas temperature after the supplementary firing, °C
Considered values
0, 5, 10, 15, 20, 25, 30 and 35
675, 645, 615, 585, 555 and 525
The parametric study was carried out following these stages:
• Stage 1. Selection of the installation’s thermal scheme. In this case the choice was a typical scheme of a
combined cycle plant for the generation of a net electric power of 600 MW;
• Stage 2. The thermal scheme was drawn using the Gate Cycle software. It includes the components geometric
distribution and their interconnection;
• Stage 3. Data input. Data input. It refers to the data input of all the components that form the thermal scheme:
gas turbines, HRSGs surfaces, steam turbine, condenser, equipment and auxiliary sub-systems, etc.;
• Stage 4. The scheme and the input data must be fitted to ISO conditions. In this stage, the program was run
several times and execution, thermodynamic, physic and geometric data and mathematic parameters errors were
analyzed. The main goal of this stage is the make the scheme ready for the off-design simulation;
• Stage 5. Simulation of the off-design operation for each of the different specified ambient temperature values,
maintaining the gas temperature after the supplementary firing within project conditions (675 °C);
• Stage 6. Simulation of the off-design operation for each one of the different ambient temperature values, varying
the gas temperature after the supplementary firing according to the specified values. As a result of stages 5 and 6,
48 operation variants were attained;
• Stage 7. Verification of results. The analysis of the limit values of different variables was carried out. This
analysis aims at avoiding reaching risky values that could jeopardize the operation reliability, for example, steam
high temperature at the high-pressure turbine inlet, etc.;
• Stage 8. Elaboration of graphics to analyze the results.
• Stage 9. Result analysis. It will be discuss in the next item.
4. Results analysis
In order to evaluate the performance of a combined cycle plant during off-design operation, because of changes in
the ambient temperature and the gas temperature after the supplementary firing, variations in generated power,
efficiency and heat rate were analyzed.
The combined cycle plant’s net thermal efficiency ‘ηNet’ is given by:
η Net =
( WGT1[kW ] + WGT 2 [kW ] + WST [kW ] − WAUXILIARY[kW ])
⋅ 100 , %
FUEL [kg / s] ⋅ LHV[kJ / kg ]
m
(1)
Where ‘W’ represents the gross power generated by the gas turbines ‘GT1’ and ‘GT2’, the steam turbine ‘ST’, and
’ represents the fuel mass flow consumed in the plant and ‘LHV’ its
the BOP power consumption ‘AUXILIARY’. ‘ m
Low Heating Value.
The combined cycle plant’s net heat rate ‘HRNet’ is given by:
HR Net =
3600.00
, kJ/kW-h
ηNet [%]
(2)
Where the net thermal efficiency ‘ηNet’ is calculated according to equation (1).
Figure 4 shows the variation in the net power generated in the combined cycle plant in relation to the gas
temperature after the supplementary firing for the different ambient temperature values that were analyzed. According
to this figure, the results of the simulation are:
• It is observed that for any ambient temperature the operational performance tendency is the same: The rise in the
temperature of the gas after the supplementary firing increases the power generated in the plant. This behavior is
caused by the increase in the power generated in the steam turbine;
• It is also observed that for the gas studied temperature range after the supplementary firing it is possible to
generate up to 70 MW more power in the installation;
• The change in the studied parameters has a significant influence on the power that can be generated in the
installation. For the maximal ambient temperature and the minimal gas temperature after the supplementary
firing, the generated power is 468 MW, whereas for the minimal ambient temperature and the maximal gas
temperature after the supplementary firing, the generated power is 642 MW, that is, a variation of 170 MW in
the generated power was registered.
650
550
Net p o wer, M W
600
500
450
525
555
585
615
Temperature after the supplementary firing, °C
645
Ta = 0 °C
Ta = 5 °C
Ta =1 0 °C
Ta = 15 °C
Ta = 20 °C
Ta = 25 °C
Ta = 30 °C
Ta = 35 °C
675
Figure 4. Net electric power generated
Figure 5 illustrates the variation in the net power generated in the gas and steam cycle of the combined cycle plant
in relation to the ambient temperature. The dark part of the graphic shows the net power generated in the gas cycle,
whereas the gray part shows the net power generated in the steam cycle. The evident result is that the sum of the two net
powers generated in both cycles is equivalent to the net power generated by the combined cycle plant. In this graphic,
the eight peaks that were observed correspond to the condition of maximal gas temperature after the supplementary
firing for each one of the values of ambient temperature that were analyzed, whereas the valleys correspond to the
condition of minimal gas temperature after the supplementary firing. The following observations can be made according
to this figure:
• The strong influence of ambient temperature produces a fall in the power generated in the gas cycle from 380
MW to 305 MW, that is, 75 MW approximately;
• By using fuel supplementary firing, for the whole range of temperatures after the firing that was analyzed, it is
possible to have a power gain in the steam cycle of approximately 77 MW;
• So, it is possible to say that the use of supplementary firing in a combined cycle installation allows a significant
compensation for the falls in the power generated in the gas cycles caused by variations in the ambient
temperature.
700
600
Net power, MW
500
400
300
200
100
0
0 -------------------------------------------- Ambient temperature, °C ---------------------------------------------- 35
Gas cycle net electric power
Steam cycle net electric power
Figure 5. Net electric power generated in the gas and steam cycles
Figure 6 shows the region where the combined cycle plant’s thermal efficiency value can be found in relation to the
variables considered for this study. According to this figure, the following was observed:
• Regarding ambient temperature, the highest efficiency values are registered when the ambient temperature is
lower. This is the result of an increase in the power generated by the gas cycle because of the ambient
temperature reduction. For a temperature of 0 °C the efficiency variation lies in a range between 53.5 % and 55.5
%;
• In relation to the temperature of the gas after the supplementary firing, the highest efficiency values were
registered when the gas temperature after the supplementary firing is smaller. This behavior takes place due to a
reduction in fuel consumption in the HRSG burners in order to achieve a lower gas temperature. Supplementary
firing causes, in average, a fall in efficiency of 1.5 percentage points for any of the ambient temperature values
that were studied;
• It is possible to say, as a result, that the combined cycle thermal efficiency variation lies within the range of 52 %
and 55.4 %, that is, it is 3.4 percentage points approximately.
55.5
55.0
Tambient = 0 °C
54.0
53.5
Efficien cy , %
54.5
53.0
Tambient = 35 °C
52.5
52.0
525
555
585
615
645
675
Temperature after the supplementary firing, °C
Figure 6. Net efficiency.
Figure 7 shows the results of the parametric study as a function of the analyzed variables , highlighting the effect of
supplementary firing. According to this figure it is possible to make the following observations:
• While the ambient temperature rises, the net power generated in the combined cycle thermal plant decreases in
spite of the use of the maximal supplementary firing temperature. It was registered that with a gas temperature of
675 °C after the supplementary firing, the net electric power varies in a range from 640 MW to 540 MW when
the ambient temperature varies between 0 °C and 35 °C;
• While the ambient temperature rises, the combined cycle thermal plant’s heat rate increases (that is, the
efficiency decreases), in spite of the use of the minimal supplementary firing temperature. The heat rate value is
even greater when the maximal supplementary firing temperature is used.
660
6950
6900
6850
610
6750
560
6700
6650
6600
510
6550
6500
460
6450
0
5
10
15
20
25
Ambient temperature, °C
Maximun temperature after the supplementary firing
Minimiun temperature after the supplementary firing
Figure 7. Power and heat rate
30
35
Heat R ate, kJ/kW-h
Net power, MW
6800
5. Conclusions
The parametric studied carried out to evaluate the influence of the ambient temperature on the operation and
performance of combined cycle plants allows the following conclusions to be expressed:
• The ambient temperature has a significant influence on this type of generating unit. Within the range of values
that was analyzed, a variation in the net power in the gas cycle of approximately 75 MW was registered;
• The ambient temperature and the temperature of the gas after the supplementary firing produce different effects
on the combined cycle plant’s performance. A drop in the ambient temperature increases the electric power
generated in the plants and its efficiency as well, and vice-versa. The rise in the temperature of the gas after the
supplementary firing increases the generated electric power but reduces efficiency, and vice-versa. The variation
of these two variables led to a variation in the generated net electric power of 170 MW, and a variation in
efficiency of 3.4 percentage points;
• The fuel supplementary firing is a technological alternative that can mitigate the power reduction in the gas cycle
caused by a rise in the ambient temperature. However, it is evident that this alternative utilization reduces
thermal efficiency. This way, the project of combined cycle thermal plants with supplementary firing in HRSGs
is restrict to installation where expenses with investment and fuel are lower than the income attained by selling
the surplus generated energy. In other words, the use of supplementary firing must be analyzed taking the
economic context into account. This way, it is possible to compare its positive effects (mitigation of power
reduction due to a rise in ambient temperature and/or the generation of surplus power for the market) with its
main negative effect that is the fall in the cycle’s efficiency.
Finally, it is important to highlight that the operation of combined cycle thermal plants, is very complex, and that
this study is far from establishing definitive criteria about it. In this sense, the researchers of Thermal Systems Study
Group will continue carrying out studies in order to find a better understanding of the variables that may have influence
on the operation and performance of this type of generating unit. In the future, they will be studied both from a
thermodynamic and an economic point of view. Other types of configurations that may become the representatives of
new thermal plants installed in Brazil and real cases based on the existing plants will also be studied.
6. Acknowledgements
The authors would like to thank the CNPq (National Council for Scientific and Technological Development) for the
financial support granted for this study.
7. References
Boyce, M., 1999, “Performance monitoring of large combined cycle power plants”, PWR- Vol. 34, Joint Power
Generation Conference, Vol. 2, ASME, pp.183-190.
Chase Jr., M. W., 1998, “NIST-JANAF Thermochemical tables”, Fourth edition, American Institute of Physics.
Gas Turbine World Handbook, 1998, Pequot Publishing, Fairfield, CT, USA.
Kehlhofer, R. H.; Warner, J.; Nielsen, H.; Bachmann, R., 1999, “Combined Cycle Gas-Steam Turbine Power Plants”,
USA, Ed. Pennwell, USA, 288 p.
Reynolds, W. C., 1979, “Thermodynamics properties in SI”, Stanford University Department of Mechanical
Engineering.
8. Copyright Notice
The authors are the only responsible for the printed material included in his paper.